1. Field of the Invention
The invention relates to a hub for measuring the torque of a cycle wheel, as well as a cycle wheel equipped with such a torque-measuring hub. The present invention also relates to a system for measuring the power of a cycle wheel, as well as a cycle wheel equipped with such a system. Finally, the invention relates to a method for measuring, using such a hub or such a system.
2. Background Information
Measuring the torque and driving power of a cycle wheel is of interest in a number of cases, for example in evaluating the performance of a cyclist during practice. The power generated by the cyclist can be determined by multiplying the cycle speed by the driving force, transmitted by the cyclist to the drive wheel. However, measuring this force is relatively complex and, in the context of the present invention, the power is determined by multiplying the driving torque, transmitted by the cyclist to the drive wheel, by the angular velocity of the wheel.
The patent document EP-0 841 549-A1 discloses a hub for measuring the power of a wheel cycle. This hub is equipped with two systems for measuring its angular position, each comprising a cam provided with teeth spaced apart angularly at regular intervals. Two switches are each provided with a metal blade in engagement with the teeth of one of the cams. The driving torque transmitted by the cyclist to the wheel is determined by comparing the time offset between the measurements of the switches. To calculate the power, the driving torque is multiplied by the angular velocity of the wheel, determined by means of the switches. Such a measuring system incorporates a large number of moving parts, which makes it relatively expensive and complicated to manufacture, and causes measurement inaccuracies.
The patent document EP-1 082 592-A1 and family member U.S. Pat. No. 6,418,797-B1 disclose a two-part hub for measuring the power of a wheel cycle. A driving part of the hub, rotationally movable about a fixed axis of the wheel, is rotationally driven by the cyclist who pedals via a freewheel device. The driving part is housed within a driven part of the hub. The driving and driven parts are radially separated by ball bearings. A connecting element supporting strain gauges is fixed to both the driving part and the driven part of the hub. Thus, when in use, the connecting element transmits the driving torque generated by the cyclist to the driven part, and the connecting element strains. The strain of the connecting element is measured by the strain gauge and makes it possible to obtain a measurement of the driving torque generated by the cyclist. Furthermore, a sensor measures the angular velocity of the drive wheel, which makes it possible to obtain a measurement of the power developed by the cyclist, by multiplying the driving torque by the angular velocity. Such a device is heavy and bulky, as it requires the presence of two concentric hub portions mutually isolated in rotation by a large ball bearing weighing down the system or, in another embodiment, by a thermoplastic bearing generating friction and hysteresis in the torque measurement, significantly degrading the measurement accuracy. This device also requires a relatively massive connecting element forming the test body supporting the strain gauges. Furthermore, the various gauges are glued to various locations of the connecting element and they must be connected to one another by wires that are also glued to the connecting element. The fixing of separate components to the connecting element poses problems that affect manufacturing quality and electronic reliability.